Solid state electronic devices, such as integrated circuits, are often housed in closed metal-walled containers or packages. The package is typically hermetically sealed and protects the electronic devices from the external environment, which sometimes contains radiation, corrosive gases or other material harmful to the confined electronic device. RF feedthroughs are used to carry RF signals through the package's metal wall between the package interior and exterior for connection to external devices. In essence, the feedthrough is a very short RF transmission line and is the conventional means to propagate RF energy through an RF barrier, such as a metal wall.
Typically feedthroughs have been constructed of glass and metal, the glass, referred to as a glass bead, located in a hole in the package wall, serves as an insulating support and dielectric that maintains a straight metal pin, the transmission line conductor, in insulated relationship with the metal package walls and as an impervious barrier to the external environment. In some instances, a single glass bead may support multiple pins.
Glass-to-metal feedthroughs of various sizes, shapes, and pin configurations have been known to the industry for over fifty years. Such feedthroughs have been formed directly within the metal wall of the package, where the wall is constructed of a nickel, cobalt and iron material, such as Kovar. They have also been constructed within a tubular ferrule of Kovar for later assembly into the package wall. The Kovar ferrule is inserted within a cylindrical aperture in the package's metal wall and soldered in place to form a relatively impervious seal.
Glasses that are highly resistive to attack by atmospheric gases, such as H.sub.2 O vapor and carbon dioxide, chemical fumes and industrial vapors are well known. One such type of glass is borosilicate glass, such as Corning 7052, a Kovar matched glass having thermal expansion characteristics closely matched to that of Kovar, Corning 7070, a Tungsten matched glass, both marketed by the Corning Company, and Kimble EN-1 marketed by the Kimble company.
In the feedthrough's construction borosilicate glass is reflowed between the central metal pin, typically a pin formed of Kovar material, and the outer ferrule. In reflowing, the melted glass forms a glass meniscus about a portion of the pin's length. Upon hardening the glass forms a strong environmental seal that resists moisture, oxidation and other harmful chemicals that might attack the integrated circuits.
A measure of the feedthrough's integrity is obtained by subjecting the feedthrough to a hermetic leakage test. In that test helium gas is placed in the sealed metal package or other enclosure in which the feedthrough has been mounted and a helium mass spectrometer type leak detector is used to detect the rate at which Helium atoms pass through the glass to metal seals, due to a defect in the glass. An acceptable package according to industry standards is one that has a helium leak rate of less than 1.times.10.sup.-8 ATM-cc/sec. He, no matter how many feedthroughs the package contains. A good individual seal should have a leak rate no greater than 1.0.times.10.sup.-10 ATM-cc/sec. He.
The performance of the borosilicate glass-to-metal feedthroughs has been well demonstrated in the industry. Presently, microelectronic packages using those feedthroughs are routinely fabricated having Helium leak rates of only 1.times.10.sup.-10 atm cubic centimeters per second.
Despite its effectiveness, glass-to-metal seals suffer a drawback. They are not durable. The glass is brittle. If the feedthrough's glass encased metal pin is deflected, bent or deformed during handling or testing, glass particles are broken at the glass meniscus surrounding the pin. That breakage compromises the integrity of the feedthrough. In some cases, radial cracks or circumferential cracks appear in the glass. Those cracks might be due to differences in thermal expansion characteristics between the glass and the pin, or some form of fatigue or from other causes, which remain unknown.
However, once even a small crack appears, the crack may propagate with repeated thermal cycling as occurs during normal use of the electronic apparatus containing the package. Once crack propagation occurs, mechanical movement of the package or mechanical stresses resulting from handling, shipping, aircraft or spacecraft vibration may aggravate the cracks and the feedthrough begins to noticeably leak. Atmospheric gases may then enter the package and damage the internal integrated circuits. Even if the initial crack in the glass does not penetrate the glass seal, the crack can expose a good portion of the length of the metal pin. When that occurs, subsequent chemical attack may corrupt the remaining portion of the pin and, ultimately, breach the seal and destroy the package integrity. Being aware of the glasses fragility, those skilled in fabricating devices containing those RF feedthroughs necessarily take extra care in handling to ensure the integrity of the product. One might hope for a more dynamic and cost efficient assembly process as would be possible if the glass seals did not require such careful handling.
In addition to its fragility, the glass seal structure is more "lossy" in its electrical characteristics than one would desire, principally due to the use of Kovar material for the central pin. Kovar is a poor electrical conductor; it is made acceptable in the glass feedthrough only by plating the exposed portion of the pin's exterior surface with higher conductivity metals, such as a layer of Nickel followed by an overlayer plating of Gold. Unfortunately, in order to form the hermetic glass-to-metal seal, the Kovar must be oxidized at those portions that are to contact the glass to allow wetting by the borosilicate glasses. That oxide surface further compromises the glass feedthrough's electrical conductivity, forcing significant restriction of current passing through the feedthrough's glass bead portion.
The pin's conductivity is dependent upon the "skin effect", described in the transmission line literature and well-known to RF engineers. That effect forces most of the current to flow essentially along the exterior surface of electrical conductors, with the electrical fields extending only a short depth below the surface. Because of that phenomenon, gold, which is highly conductive, plated on another conductor, such as Kovar, provides an excellent conduit for conduction. At RF frequencies above 20 Ghz, the skin effect is more pronounced, concentrating the RF fields at the surface and a minute depth into the conductor. Because the Kovar pin is plated with a layer of Gold, the bulk of the RF transmission takes place principally in and along the Gold plating and not significantly in the underlying highly resistive Kovar. For that reason it is permissible to use Kovar material as part of an RF transmission medium without the RF signal encountering significant resistive losses. However, in the glass to metal seal, only the portions of the Kovar pin that lie outside the glass bead may be gold plated to enhance the pin's electrical conductivity. The central portion of the Kovar pin that fits through the glass bead, however, cannot be Gold plated for the reason earlier stated and, therefore, compromises the electrical conductivity of the transmission path. The glass-to-metal feedthrough thus exhibits low electronic efficiency.
Accordingly, a principal object of the present invention is to improve the electronic efficiency of RF feedthroughs by enhancing the feedthrough's electrical conductivity.
A further object of the invention is to provide RF feedthroughs that are physically more hardy and durable than the glass-to-metal type by eliminating glass from the feedthrough structure.
A still further object of the invention is to provide a new feedthrough structure that may employ metals of higher electrical conductivity than Kovar.
An additional object of the invention is to increase the efficiency with which RF feedthroughs may be installed in electronic equipment.
A still additional object of the invention is to provide a glass-less RF feedthrough that has a helium leak rate of less than 1.times.10.sup.-10 atmospheres cubic centimeters per second and is of greater durability than the glass-to-metal type feedthroughs.
And an ancillary object of the invention is to provide a new feedthrough structure whose center pin may be bent or straightened as desired, without damaging the feedthrough's hermetic seal.